Carburettor

Question 1

Explain the following in relation to fuel-air mixture ratios:

(a) rich and lean;
(b) normal workable mixture ratio limits;
(c) the chemically correct or stoichiometric ratio;
(d) the approximate ratios for maximum power output and best economy.

Answer 1

(A) means more or less fuel in a mixture
(B) 1:9 (rich) - 1:18 (lean)
(C) 1:15
(D) 1:12 max power, 1:16 ideal

Question 2

Explain the basic principle of operation of a simple float-type carburettor

1. Venturi
2. Fuel jet
3. After the fuel jet

Answer 2

a Venturi for air to flow through the fuel-air mixture. When air flows through airflow must accelerate and therefore experiences a drop in pressure (Bernoulli’s theorem).
Proportional to the rate at which the air is allowed to flow through the throat of the carburettor, which is in turn controlled by throttle butterfly valve.

Main fuel jet of the carburettor is positioned in the low-pressure area at the centre of the Venturi. When air flows through Venturi, the difference in pressure between the air above the fuel in the float chamber and the pressure at the outlet, results in fuel being drawn from the jet ( like a straw.

After it leaves the jet, liquid fuel vaporises and mixes with the air on its passage past the throttle butterfly and through the inlet manifold the cylinders.

Question 3

Explain the use and the need for the following in an aero-engine carburetor; atomisation and diffusion system

Answer 3

Main metering system delivers fuel into the intake air at all engine speeds above idling.

The same principle as fuel nozzle of the simple carburettor but the design is refined to include air bleed for providing a fuel-air emulsion and a diffuser at the nozzle.

Overall improves vaporisation and mixing of the fuel with the inlet air.

Question 4

Explain the use and the need for the following in an aero-engine carburettor; accelerating system

Answer 4

Required to supply additional fuel during increases in engine power, without one ant rapid opening of the throttle will cause engine to falter or cut out altogether. (USE)

when throttle is open, airflow increases through the manifold rapidly but due to inertia there is a delay before the added proportion of fuel can be supplied through the main metering jet. Leading to temporary leaning of the mixture. (NEED)

Question 5

Explain the need and the working of an idling system

Answer 5

Low rpm, air being drawn from Venturi is decreased and pressure drops so fuel can’t be sucked out of main nozzle.

results with the main metering system dropping to zero or becoming unreliable.

Idling works as throttle valve is opened, the gap between it and the carburettor increases, the effectiveness of the idling system reduces, while at the same time the pressure signal of the main metering system improves.

Question 6

Explain the need for a power enrichment system

Answer 6

Required at high manifold pressure and rpm to avoid detonation and overheating.

Done by bringing a second metering jet into operation and increasing volume of fuel flow for a given metering signal when throttle valve is at or near wide open end of its range.

Question 7

Explain the use and the need for the following in an aero-engine carburettor; mixture control

Answer 7

Mixture control is used to properly adjust the fuel-air ratio to suit the operating environment of the engine at any time.

Question 8

Explain the use and the need for the following in an aero-engine carburettor; idle cut off

Answer 8

Enables the engine to be stopped by cutting off the fuel supply at the carburettor. Ensures cylinder contains no unburnt mixture or is left too lean. Also leaves the carburettor full of fuel for the next start.

Question 9

Disadvantages of float type carburettor

Answer 9

Fuel flow can be disturbed by manoeuvres that upset float mechanism, especially those that are less than one g.

Susceptibility to icing ( specifically refrigeration)

Question 10

Explain correct use of mixture control

Answer 10

All stages full rich except leaned on cruise

Question 11

Explain the consequences of operating with over-rich and over-lean mixture settings

Answer 11

Over rich
Loss of power
Rough running
Spark plug fouling

Over Lean
High CHT
Detonation

Question 12

How and when is refrigeration icing formed?

Answer 12

When moisture content of the air freezes as a result of cooling caused by fuel vaporisation.

Occurs generally 4 - 25 degrees
Occurs whenever RH is more than 50%

Question 13

How and when is throttle icing formed?

Answer 13

When water vapour condenses and freezes due to the Venturi effect when fuel-air temperature is less than 0º

Forms with outside air temperature up to 25º with RH greater than 50%

Question 14

When and how does impact icing form?

Answer 14

When ambient temperature is at or below 0º and aircraft is flying through visible moisture.

Supercooled water droplets freeze on impact

Question 15

Explain the symptoms of carb icing.

Answer 15

Fixed pitch propellor - reduction in RPM
Constant speed- reduction in manifold

Rough running, vibration, further loss of performance, engine stops due to fuel starvation.

Question 16

Explain the correct use of carb heat control.

Answer 16

Drop initially in RPM or MAP, test on ground for run ups. No drops = faulty, no ice protection don’t fly.

As ice melts the RPM/MAP increases, when returned to normal carb heat OFF.

Question 17

How does carb heat work?

Answer 17

Hot cured air from around the engine exhaust which is unfiltered.

Question 18

Describe the effect of excessive carburettor heat in high output engines.

Answer 18

Too much carb heat may lead to detonation, particularly in higher powered engines, due to engines operating at higher temperatures.

Hotter air may raise temp leading to potential detonation.

Question 19

Explain the reasons for a reduction in power when carburettor heat is operated

Answer 19

Hot air is less dense, so there may be an rpm drop of 10-20%

Question 20

Describe the typical source of heat for the carburettor hot air.

Answer 20

Hot ducted air from around (not from inside!) the engine exhaust

Question 21

With respect to carburettor air intakes, explain the correct use of:

(a) ram air;
(b) filtered air;
(c) carburettor heat.

Answer 21

A. Ram air intake works by reducing the intake air velocity by increasing the cross-sectional area of the intake ducting
B. Air that passes through the carburettor and an air filter making it clean
C. Normally air enters carburettor before passing through a filter and carb heat is usually not filtered, limited use on the ground.

Question 22

Explain the principal differences between a fuel injection system and carburettor systems

Answer 22

Fuel injection systems atomise fuel and mix it in the correct proportion with air.

carburettor relies on suction created by intake air accelerated through a Venturi to draw fuel into the airstream.

Question 23

Explain the purpose of the following components in a basic fuel injection system:
(a) delivery pump system; engine driven pump

Answer 23

engine driven pump is used to control and maintain a positive pressure on the fuel delivered to the metering unit by feeding excess fuel through a fixed orifice in a return line to the fuel tank.

For direct fuel injection systems the pump divides fuel into equal portions and its timed to force this amount of fuel at high pressure through the discharge nozzle into the cylinder.

Question 24

Explain the purpose of the following components in a basic fuel injection system: distribution system

Answer 24

Function is to deliver correct amount of fuel to the injection pump depending on the amount of air it senses is passing through the throttle unit.

Fuel control unit filters the fuel and then passes it first through the mixture control valve then metering valve.

Question 25

Explain the purpose of the following components in a basic fuel injection system: injectors

Answer 25

Continuous flow - inject fuel into inlet ports of each cylinder.
Amount of fuel depends on RPM and mixture controls

Direct - spring loaded valve, designed to open only when a high pressure is applied ensuring a clean start to the injection.

Question 26

Explain the principal differences between continuous flow fuel injection, and direct fuel injection systems

Answer 26

Direct fuel is injected directly into each cylinder in a pulsing fashion whereas with continuous fuel flows from the injectors at all times. Flow rats can be changed on power output and air pressure.

Question 27

With regard to using fuel injection systems in aero-engines, explain the:
(a) advantages (6)

Answer 27

• No refrigeration ice
  •More uniform delivery of fuel-air mix to cylinder
  •Improved control of fuel-air ratio
  •Fewer maintenance problems (no pesky carburetor)
  •Better throttle response
  •Increased engine efficiency

Question 28

Disadvantages of fuel injection

Answer 28

Vapour lock in the fuel lines
which may make restarting a hot engine difficult (or impossible until the vapour lock clears)

Susceptible to fuel contamination from water or small debris
as the fuel lines are very small in diameter

Pilot needs a greater awareness of fuel distribution
As surplus fuel from the fuel pump is generally
only returned to one tank

Question 29

Explain the normal symptoms of intake ice formation, and the correct use of alternate air sources.

Answer 29

Likely to occur when the ambient temperature is at or below 0°C and…
•The aircraft is flying through visible moisture
•Can affect fuel injected and carburetered systems

Alternate air

Used when air filter for the carburettor air becomes partially blocked with foreign material meaning less air to get into the engine. So alternate air is used if this occurs to prevent engine failure.

Question 30

With regard to exhaust driven turbo charging and mechanically driven supercharging, explain the:
(a) advantages; (5)

Answer 30

Higher power output (for a given density altitude)
•Better atomization and mixing of fuel
•Faster acceleration
•Better scavenging of waste gases
•Reduced Specific Fuel Consumption

Question 31

Explain the basic principle of operation of an exhaust driven turbocharger.

Answer 31

The turbine of the turbocharger is driven by the exhaust gases of the engine. Control of the speed of the turbine is dependent on the position of the waste-gate control valve
– Fixed waste-gate
– Variable waste-gate

• A variable gate allows the pilot to vary the amount of exhaust gases that enter the turbine – so can spin it faster or slower
• If it spins faster, it compresses more air – needed at higher altitudes

Question 32

Disadvantages of turbochargers (3)

Answer 32

Backpressure in the exhaust system, leading to reduced efficiency
•Increased charge (fuel & air) temperature – may contribute to detonation
•Increased Friction loss – impellor

Question 33

How does a supercharger work

Answer 33

Impeller is driven by the crankshaft directly or by a reduction gearbox.

Question 34

Disadvantages of superchargers (4)

Answer 34

Use engine power for operation (as driven off the crankshaft)
•Higher fuel consumption
•Increased charge (fuel & air) temperature – may contribute to detonation
•Increased Friction loss – impellor

Question 35

Describe the function of the exhaust man

Answer 35

Has the function of collecting the products of combustion form each of the cylinders and discharging them into the airstream through the exhaust outlets.

Question 36

Explain the importance of proper sealing of the exhaust manifold.

Answer 36

Leaking exhaust manifold presents a fire hazard and can lead to toxic gases entering the cockpit like carbon monoxide which is colourless and odourless

Question 37

Describe the possible sources, indications and associated danger of carbon monoxide ga

Answer 37

Check CO detector and see if there are any black dots on the orange circle. Signs include sleepiness, headache and hypoxia